Aluminum nitride-based semiconductor devices including an AlGaN light-emitting layer are compound semiconductor devices that become capable of emitting light primarily in the near ultraviolet region through adjustment of the aluminum composition of the light-emitting layer. Ultraviolet light is an electromagnetic wave with wavelengths ranging from 10 nm to 400 nm. In particular, the ultraviolet light with wavelengths from 200 nm to 400 nm is called near ultraviolet light (NUV). In some cases, near ultraviolet light is further divided into UVA (between and including 315 nm and 400 nm), UVB (longer than or equal to 280 nm and shorter than 315 nm), and UVC (shorter than 280 nm). In addition, the ultraviolet light with wavelengths of 300 nm or shorter is sometimes called deep ultraviolet light (DUV). By using an aluminum nitride-based semiconductor with an AlGaN light-emitting layer and suitably increasing or decreasing the Al composition of the layer, a semiconductor light-emitting device can be manufactured that is theoretically capable of emitting near ultraviolet light with wavelengths from 192 nm to 365 nm. Examples of conventional semiconductor light-emitting devices based on a semiconductor material include light-emitting diodes (LEDs), semiconductor laser diodes (semiconductor LDs), and superluminescent diodes (SLDs).
Semiconductor light-emitting devices manufactured using aluminum nitride-based semiconductors are expected to replace, for example, conventional ultraviolet light lamps and sources and to find applications in the fields of, for example, disinfection, resin curing, and medical procedures, by relying on their light emission capability in the ultraviolet region (especially, in the deep ultraviolet region of 300 nm or shorter wavelengths). Today, research and development is actively pursued, and small-scale manufacturing of deep ultraviolet LEDs has already been started.
Deep ultraviolet LEDs developed so far (wavelengths of 300 nm or shorter), however, have a light emission efficiency of approximately a few percent, which is far lower than the efficiency of the commercialized blue LED (which is a few dozen percent). This is attributable to factors such as the physical constants of the aluminum nitride-based semiconductor itself used in the deep ultraviolet LED, factors such as crystal growth and other device manufacturing technology, and combinations of these factors.
Examples of the physical constant-related factors include (i) very low light extraction efficiency in guiding the ultraviolet light generated in the light-emitting layer out of the semiconductor due to the low refractive index of the aluminum nitride-based semiconductor and (ii) high likelihood of absorption by substances inside and outside the device due to short, ultraviolet region emission wavelengths. Examples of the device manufacturing technology-related factors include the need for advanced technology to produce high quality crystal with high yields and high reproducibility because the aluminum nitride-based semiconductor is manufactured at as high as 1,000° C. or at even higher temperatures. Examples of the combined factors include the fact that AlGaN, if containing more than 25% Al, is difficult to render p-type even with high-concentration p-type doping because of high acceptor activation energy and also the fact that high p-type carrier density is difficult to achieve due to, for example, difficulty in growing AlGaN crystals and resultant incomplete crystallinity of AlGaN and under the influence of, for example, residual donors produced by contamination with unintended oxygen and other impurities.
An aluminum nitride-based semiconductor light-emitting device is manufactured typically by stacking many semiconductor layers on an underlayer substrate such as a sapphire substrate using MOCVD apparatus or like crystal growing apparatus. A bulk AlN substrate may be used as the underlayer substrate.
When a sapphire substrate is used as an underlayer substrate on which crystal is to be grown for an aluminum nitride-based semiconductor layer, it is difficult to form a high quality semiconductor layer because numerous crystal defects and cracks occur in the aluminum nitride-based semiconductor layer due to a difference in lattice constant between the sapphire substrate and the aluminum nitride-based semiconductor layer. A method addressing this problem is publicly known where an AlN buffer layer is formed on a sapphire substrate before sequentially stacking an n-type AlGaN contact layer, a light-emitting layer, a p-type contact layer, and other layers to obtain a high quality growth layer. Another method is also publicly known where AlN, which exhibits a lattice constant that is close to that of the AlGaN layer serving as a light-emitting layer, is used as a substrate for high quality crystallinity to obtain a high quality growth layer. Various research and development activities are also being undertaken in research institutions to find a method to increase light extraction efficiency.
When a sapphire substrate is used as an underlayer substrate on which crystal is to be grown for an aluminum nitride-based semiconductor layer, the lack of electric conductivity of the sapphire substrate necessitates a parallel and lateral electrode structure where materials need to be removed by dry etching down to the n-type layer to form an n-type electrode. However, in the parallel electrode structure, light emission efficiency drops significantly due to current concentration. Blue LEDs and other light-emitting devices that use an InGaN-based semiconductor have the same problem of low light emission efficiency caused by current concentration. Aluminum nitride-based semiconductors, both p- and n-types, have a far higher resistivity than InGaN-based semiconductors and are hence more susceptible to current concentration-caused low light emission efficiency than blue LEDs.
To solve these problems that result from the lateral structure, methods are being investigated of detaching the underlayer substrate (e.g., sapphire substrate) after an aluminum nitride-based semiconductor layer structural body is attached to a conductive support substrate.
For the blue LED, technology is established where a semiconductor light-emitting device is manufactured by bonding a support substrate to a semiconductor layer grown on an underlayer substrate and thereafter removing the underlayer substrate (sapphire substrate) and other related elements, as described in Japanese Unexamined Patent Application Publication, Tokukai, No. 2009-231560 (Patent Literature 1) and Japanese Unexamined Patent Application Publication, Tokukai, No. 2014-38920 (Patent Literature 2).